To protect the occupants of a motor vehicle in accidents, seats of the motor vehicles have been provided not only with reversible restraint arrangements such as safety belts, but also with irreversible restraint arrangement such as airbags, which are triggered, for example pyrotechnically fired, when a collision is detected.
However, faulty firing or firing at the wrong time, even if an accident has occurred, may result in serious injuries that would not occur if the airbag were not fired or, in the case of multistage airbags, were fired in some other way. Furthermore, unnecessary firing must be avoided, especially when the seat is not occupied or is occupied only by an object and not a person.
U.S. Pat. No. 5,570,903 discusses systems for detecting child seats by sensors arranged in the seat surface, for example, or to detect the occupation state, as in U.S. Pat. No. 5,983,147, by video monitoring, for instance.
However, this is still not sufficient. Instead, to avoid disadvantageous and sometimes even dangerous triggering in particular situations, it must be ascertained whether a person occupying the seat is tall or short, heavy or light, is close to the instrument panel or is leaning back in the seat. Depending on the individual situation, the restraint arrangement must be triggered in a variety of ways once an accident occurs, which applies in particular to an airbag able to be fired in several stages, again especially in the case of an airbag for a front-seat passenger. A division into groups, i.e., a classification, has shown to be sufficient.
For this purpose, the so-called OC system (occupant classification system) has been developed. This system is based on an empirically determined correlation between the body weight and the spacing of the ischiadic tubers of a person. As a result, a pressure profile may be acquired and analyzed with the aid of pressure sensors, which are arranged in a matrix-like manner in a seat. This analysis, first of all, also allows or provides for detecting whether or not the seat is occupied. Furthermore, a distinction may be made whether a child seat or some other object or a person is in an occupied seat.
If a person has been detected as object, a further classification may take place by a corresponding analysis of the pressure profile on the basis of empirical findings that correspond to the body size and the body weight of the seated person. Furthermore, using suitable sensors, the absolute weight of the object in the seat may be detected. For example, the weight of the seat with the object may be measured with the aid of foil strain gauges, for instance. On the other hand, the pressure differential between an occupied and an unoccupied seat using a sensor such as a pressure foil, installed in the seat itself, may be detected, thereby detecting the absolute weight of the object.
The conventional classification utilizes empirical findings, which in individual cases, which are of interest here, may provide wrong results in the control of the triggering of the restraint arrangement, however.